A chain saw's components include, a loop of saw chain consisting of interconnected links, some of which carry cutting teeth, a guide bar that guides and supports the loop of saw chain for rapid rotation around the guide bar, and a power head that drives the saw chain and also provides the handles that the sawyer grips for directing the cutting action of the chain saw. Wood is non-homogeneous and its resistance to cutting is inconsistent and therefore rough. The chain saw is necessarily a very powerful tool and one that inherently demands concern for both safe operation and cutting performance.
A primary concern of recent years is the occurrence of kickback. Kickback occurs when a cutting tooth "hooks" into a limb or log (generally while passing around the outer end or nose of the guide bar) in such a manner as to induce rearing of the chain and bar and possible injury to the sawyer. Such kickback action has been a concentrated target for design improvements and has been largely reduced by providing guard like protrusions that precede the cutting tooth.
A secondary consideration of the chain saw sawyer is the incidence of vibration. The very nature of a rotating cutting chain engaging and cutting a tree or log wherein small wood chips are rapidly ripped from the log, creates vibration. This vibration, over time, is detrimental to the sawyers health and causes what is termed in the industry as "white fingers disease". This is a numbness that appears in the sawyers hands after extended use of a chain saw.
reducing the vibration that occurs in the operation of a chain saw is the objective of Engman et al in U.S. Pat. No. 4,122,741. Engman determined that vibration was largely caused when the cutting edge engaged a hard spot in the wood, e.g., a knot. The cutting link was pivoted about its heel and because the cutting edge was forward of the heel, it pivoted deeper into the wood causing the chain to jerk or vibrate. This action was alleviated by changing the geometry of the cutting link whereby the heel was elevated off the bar so that a cutting impact caused the cutting edge to pivot around the front rivet, i.e., out of the wood.
Not disclosed or suggested in Engman is a safety link which often is added to the sequence of chain links to decrease kickback likelihood at the nose of the guide bar. On the straight reach of the bar the guard portion of the link projects toward the wood but is normally short of the cutting edge and does not cut. Its presence in the chain link sequence during a typical cutting operation nevertheless induces vibration that is not explained by the Engman theory. Furthermore, the vibration is present irrespective of the location of the guard portion over the rear rivet of the link, i.e., where rearing of the link would not increase the outward projection of the guard portion. Whereas the increased vibration is a problem for the sawyer, nevertheless the safety link is necessary for solving a problem of higher priority, i.e. reducing the likelihood of kickback. Therefore the sawyer has heretofore simply lived with the vibration caused by the safety links.